Zoom lenses having a four group configuration comprising groups of positive, negative, negative, and positive powers from the object end to the image end, respectfully, have been well known, but the designs of such lenses have been more complex relative to positive, negative, positive, positive zoom lens forms. The two negative groups move smoothly in the same direction for zooming, and the first positive group moves for focusing. Previously such zoom lenses have been directed primarily to applications in cinematography where size and performance characteristics are different than are desirable for use in 35 millimeter still photography.
One of the shortcomings in the prior zoom lenses is that the distribution of optical power among the groups does not allow for proper focusing or a suitable front vertex to film distance. In prior art zoom lens designs where the second group is a singlet of negative power, the ratio of the absolute value of the power of the singlet to the power of the first group is greater than 1.75. This ratio has been shown to be unsatisfactory for a low cost lens for 35 millimeter still photography, because of the requirement for relatively expensive glasses in the third group to correct for chromatic aberrations.
Favorable design characteristics cannot be obtained from merely reducing the power ratio of the second to the first group. For instance, it is not sufficient to reduce the power of the second group because the first group must have the proper relationship to permit close focusing.
Conventional designs of zoom lenses of the positive, negative, negative, positive group configuration have heretofore required a relatively large number of elements and have not been successfully employed in producing a suitable compact lens. Such previous zoom lenses frequently employ a rear group comprising a triplet type lens configuration. This latter configuration also tends to detract from the compactness of the lens.